Anisotropy of thermal vibrations and polymorphic transformations in lanthanum and uranium

Abstract

AbstractAverage thermal atomic displacements along different crystallographic axes were measured for polycrystalline samples employing the method of neutron diffraction and defining the anisotropic Debye–Waller factor, obtained by processing the corresponding reflections. In the double hexagonal close‐packed lanthanum modification, the anisotropy of vibrations is insignificant at room temperature, yet it increases markedly with increasing temperature; also, the relative displacement (in relation to the corresponding lattice parameters) in the basal plane considerably exceeds that along the hexagonal axis. In orthorhombic α‐uranium, large anisotropy is observed even at room temperature, which increases with increasing temperature. It is clearly proved that in lanthanum and uranium the polymorphic transition of low‐temperature anisotropic phases into high‐temperature cubic modifications with isotropic thermal vibrations reduces the maximum relative quantity of thermal displacements at the expense of more homogeneous distribution of vibrations along the lattice axes, and thus probably ‘prevents’ the melting of low‐temperature modifications at a lower temperature due to the critical value of these displacements being reached. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)